Heart disease is the leading cause of death in the United States. Every year more Americans die of heart disease than from all forms of cancer combined. Cholesterol levels are an important factor in the etiology of heart disease, and may offer a favorable target for drug-based therapies. The goal of this research is to learn how cells control the synthesis of sterols such as cholesterol in mammals and ergosterol in yeast. These studies will be done on the yeast Saccharomyces cerevisiae, but the remarkable conservation in the sterol biosynthetic pathway, and in at least some of the enzymes of this pathway, suggests that many of the lessons from yeast will be directly testable in mammalian cells. The enzyme known as HMG-CoA reductase catalyzes the rate limiting step in sterol biosynthesis, which is the conversion of HMG-CoA to mevalonate. It is known that products made from mevalonate feedback repress HMG-CoA reductase synthesis and activity. However, the identify of the active molecules is not known. The identification of these molecules is a goal of this research. knowledge about which molecules participate in feedback control should be important in devising pharmacological strategies for controlling cholesterol synthesis and treating atherosclerosis. On a more basic level, this research is likely to shed light on a novel mechanism of translational regulation that controls, at least in part, HMG-CoA reductase synthesis Intracellular heme concentration may be a major factor in this regulation since mutants deficient in heme biosynthesis have reduced levels of HMG- CoA reductase synthesis. Mutations in genes that regulate synthesis of HMG-CoA reductase have been isolated and will be studied in order to learn how these gene products monitor the need for sterol and regulate gene expression. Electron microscopy and genetics will be combined to study the biogenesis of karmellae, which are pairs of membrane layers associated with the nucleus that are proliferated upon HMG-CoA reductase overproduction. These membranes appear to be formed from the endoplasmic reticulum or the nuclear envelope. The inducible formation of karmellae offers a unique opportunity to study the synthesis and turnover of the ER-nuclear envelope in a genetically tractable organism.